Conventional loudspeakers have long relied upon the use of cone-shaped air driven mechanical elements. In such loudspeakers, each cone shaped element is mechanically driven on one end in a piston-like manner (i.e., “pistonic”) with the use of a moving coil that employs an electromagnetic drive means. This drive means includes a fixed magnet assembly mounted to a frame or chassis of the loudspeaker in a manner that ensures a strong translation of electromagnetic energy into a moving coil and cone assembly for strong and responsive drive action. Typically, lightweight sheet materials have been used in the cones of conventional loudspeakers as well as stiff composite sandwich structures that minimize bending over an operational frequency range. In conventional loudspeakers, these types of designs have generally achieved excellent results especially where different types and sizes of cone elements and associated drive units are used for different frequency ranges with appropriate electronic circuitry in the loudspeaker housing. Such designs, however, have significant disadvantages. First, their mass and bulk tend to be substantial physical limitations. Second, the sound produced from one or more cone elements is often constrained to an axial origin that imposes an unavoidably high directionality, particularly at higher frequencies.
In response to these limitations of conventional loudspeakers, a fundamentally new form of loudspeaker was developed, as described in detail in international patent application WO 97/09842 A2 and whose contents are incorporated by reference herein in their entirety, that relies upon flatter acoustic elements and/or diaphragms that that have less depth and less directionality. These types of loudspeakers have come to be referred to as “distributed mode loudspeakers” since they rely on exploiting the mechanical resonance of the panels used in these loudspeakers. In addition to their ability to generate sound relying upon mechanical resonance, such distributed mode loudspeakers are novel because of their use of materials that are capable of sustaining bending waves and their ability to generate sound from the action of those bending waves. These materials are formed in the shape of panels and have been shown to provide wide frequency coverage and robust sound distribution and loudness capabilities with wide directivity that is independent of panel size and significantly diffuse output yielding highly sympathetic boundary interactions. The drive force from transducers used in a distributed mode loudspeaker, the structure of its panel, and associated boundary conditions enables the panel to radiate sound energy with both significantly pistonic and significantly modal vibrations. Typically, at the lowest frequencies of operation, the vibration of the panel may be significantly pistonic in character, becoming progressively more modal with increasing frequency.
Although distributed mode loudspeakers are relatively new, certain key design principles have already been developed and have been adopted by designers in this field. Notwithstanding the existing understanding of these design principles, a significant design problem remains, particularly with high power distributed mode loudspeakers. At higher operational powers, the transducers used in such loudspeakers, designed according to these existing design principles, frequently suffer from persistent rocking motion at lower pistonic frequencies. This is a problem since at high powers during pistonic operation, the rocking motion of audio transducers can cause physical damage to the voice coil provided in each transducer. Typically, distributed mode loudspeakers utilize a plurality of transducers for increased modal distribution and power handling. These resonant modes are important since each one contributes a particular component of bending wave vibration action over a panel that ranges between vibrationally active subareas and vibrationally inactive areas, corresponding to “anti-nodes” and “nodes,” respectively, of the resonant modes. Therefore, a significant and rapidly growing need exists for a solution to the problem caused by the physical rocking motion of transducers when distributed mode loudspeakers are used in high power applications while operating in pistonic frequency ranges without compromising their performance benefits across their entire operational frequency range.